This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. We have developed a technique for quantitative three-dimensional mapping of refractive index in live biological cells and tissues. The projection of sample refractive index is imaged using a Mach-Zehnder phase-shifting heterodyne interferometer with multiple directions of illumination. A 3-D tomographic reconstruction is then derived from the collection of 2-D projection phase images using a filtered back-projection and iterative constraint algorithm. Refractive index maps will have applications in high resolution microscopy, cellular light scattering, and optical methods for disease diagnosis and imaging.